1. Field of Invention
This invention relates generally to steam generators for humidifying an air stream, and more particularly to a module for this purpose formed by a tank divided by a barrier wall into a water reservoir section and a steam generating section, water boiled off in the steam generating section being replenished by water drained from the reservoir section.
2. Status of Prior Art
As pointed out in our above-identified copending patent application, a proofing chamber requires a hot and humid atmosphere in order to raise dough pieces placed within this chamber preparatory to their being baked. A similar atmosphere is necessary in a holding chamber to maintain cooked food placed therein in a hot and moist state in condition for serving. To satisfy the atmospheric requirements for a proofing chamber and the different requirements for a holding chamber, it is essential that the steam source of this atmosphere be adjustable to produce an atmosphere whose relative humidity is appropriate to the chamber.
The absolute humidity of air is the weight of water in a unit volume of air. Relative humidity is the ratio, in percentage terms, of the moisture actually in the air (absolute humidity) to the moisture the air would hold if it were saturated at the same air temperature and pressure. The point at which saturation is reached represents the capacity of the air to hold water vapor. This point increases rapidly as the air temperature increases.
In a module of the type disclosed in my prior U.S. Pat. No. 5,802,963 entitled "Module for Producing Hot Humid Air," water from a reservoir is fed into a steam generator having an electric heater. Steam from this generator is injected into a duct in which a blower draws in air through a duct inlet and blows it through an air heater to produce a humid, hot air stream which is exhausted from the duct outlet. The relative humidity of the resultant stream depends on the amount of steam injected therein and the temperature of the air.
With this module it becomes possible to adjust the temperature of the air stream as well as its relative humidity. But we have found that it is not possible, when operating in conjunction with a holding chamber, to provide an atmosphere for this chamber appropriate for certain food holding conditions, such as an atmosphere whose air temperature is well above 130.degree. F. and whose relative humidity exceeds 50 percent.
More water vapor is required to produce a high relative humidity at higher temperatures. Thus if the air temperature is 130.degree. F. and a certain amount of steam is injected into the heated air to impart a high relative humidity thereto, given the same amount of steam, but an air temperature of 150.degree. F., the relative humidity will then be substantially lower.
The difficulty with the module disclosed in my prior patent is that it is unable to supply to the hot air stream whose air is at an elevated temperature the greater volume of steam needed to produce a high relative humidity. Hence the module, though capable of providing an atmosphere that is appropriate for the wide range of conditions encountered in proofing, it is unable to provide an atmosphere appropriate for a wide range of holding conditions.
In a module of the type disclosed in my above-identified copending application, the module is adapted to operate effectively in conjunction either with a proofing chamber in which dough pieces are raised prior to baking by being subjected to a stream of hot and humid air, or in conjunction with a holding chamber in which cooked food is maintained in a hot, moist state in condition for serving.
This module is provided with a water reservoir from which water is fed into a steam generator where the water is boiled to generate steam. Associated with the module is an air duct having an inlet which feeds incoming air through a blower and an air heater toward an outlet whereby air drawn into the duct through its inlet by the blower is heated to produce a stream of hot air that is exhausted from its outlet. A steam tube coupled to the steam generator injects steam into a positive-pressure zone in the duct beyond the blower therein to intermingle with the stream of hot air to produce a stream of hot and humid air. When the module operates in conjunction with a proofing chamber this stream is fed therein to effect a proofing action.
Steam from the steam chamber is also fed into a negative-pressure zone in the duct in advance of the blower therein to produce a stream of hot and humid air having a higher temperature whereby when the module operates in conjunction with a holding chamber, it then effects a holding action for the cooked food therein.
In my copending application, the module disclosed therein in which a covered water tank is divided by a barrier wall into a reservoir section and a steam generating section, is directly associated with an air duct provided with a blower and an electric heater to produce a hot air steam into which is injected steam discharged from the module.
The need exists for a stand alone, steam-generating module that need not be associated with an air duct provided with a blower and a heater, for when the module is so associated the operation of the module is limited to the air capacity of the duct. There are many practical applications for a steam generator other than for producing a hot and humid atmosphere for a proofing or holding chamber in which the basic requirement is humidification regardless of whether the air stream in which the steam is injected is at a low or high temperature. Indeed, in some applications the air stream may be air conditioned.
When a steam generator of the type disclosed in the above-identified copending patent application makes use of a conventional electric water heater unit having a metallic heater element in direct contact with water, then in the course of prolonged operation, lime and other minerals dissolved in the water are coated on and adhere to the metallic surfaces. This lime coating is thermally insulating and reduces the effectiveness of the unit. Hence when a standard electric water heater unit is mounted within the steam generating section of the module, it becomes necessary on occasion to shut down the steam generator in order to delime it.
Of prior art interest in regard to electric water heater units is the Eckman U.S. Pat. No. 5,586,214. This patent discloses an immersion heater whose electric resistance elements are encapsulated within a layer of polymeric material that is electrically insulating but thermally conductive and therefore does not thermally insulate the heating element from the water in contact with the polymeric layer.